Non-static method cannot be referenced from static context?

Question

I was watching the programming class in java and basically the business was to calculate the total area of a house with a pool, however, I’m having difficulties. Basically I separated everything into three files, being one of them the main one, but when I try to compile, appears the message saying:

non-static method cannot be referenced from a static context

Areacasa.java:

public class AreaCasa {
    //preço do metro quadrado
    double valorM2 = 1500;

    //calcula a área da casa
    double CasaRet(double l_sala, double c_quarto) {
        double area_s; //área da sala
        double area_q; //área do quarto
        double area_t = 0; //área do total

        if(l_sala < 0 || c_quarto < 0)
             System.out.println("Erro!");

        else{
            area_s = l_sala * l_sala; //calcula area da sala
            area_q = c_quarto * (l_sala/2); //calcula area do quarto            
            area_t = area_s + 2 * area_q; //calcula a area total
        }
        return(area_t);
    }
}

Areapiscina.java:

public class AreaPiscina {
    double AreaPiscina(double raio){ 
        return((raio >= 0) ? Math.PI * Math.pow(raio, 2) : -1);
    }
}

Project.java (is the main file):

public class Projeto {
    double Area(double lateral_1, double lateral_2, double pis_raio) {
        return(AreaCasa.CasaRet(lateral_1, lateral_2) + AreaPiscina.AreaPiscina(pis_raio));
    }

    public void main (String args[]) {
        System.out.println(Area(21.43, 33.4, 2.0));
    }
}

When I try to compile this last file appears:

[user@localhost TesteJava]$ javac Projeto.java
Projeto.java:3: error: non-static method CasaRet(double,double) cannot be referenced from a static context
    return(AreaCasa.CasaRet(lateral_1, lateral_2) + 
                   ^
Projeto.java:4: error: non-static method AreaPiscina(double) cannot be referenced from a static context
    AreaPiscina.AreaPiscina(pis_raio));
               ^
 2 errors

I can’t understand why it’s giving error, the functions within the design class are not even static (I think)! I think I did everything right by passing the values by reference to another function.

Answer 1

You are trying to access non-static methods statically.

In doing AreaCasa.CasaRet(lateral_1, lateral_2) + AreaPiscina.AreaPiscina(pis_raio); you are calling the methods of the classes AreaCasa and AreaPiscina statically (Class.method) without the methods being static (identified by static), not to mention you’re trying to give a return in a builder.

To use the methods the way you wrote them, you need to instantiate the classes before using your methods:

Note: The constructor does not return values, so you need to create one method other than class name AreaPiscina.

    public class AreaPiscina {

    double calcularArea(double raio){ 
      return((raio >= 0) ? Math.PI * Math.pow(raio, 2) : -1);
    }

}

And then make the call:

 double Area(double lateral_1, double lateral_2, double pis_raio) {
         AreaCasa casa = new AreaCasa();
         AreaPiscina piscina = new AreaPiscina(); 
        return(casa.CasaRet(lateral_1, lateral_2) + piscina.calcularArea(pis_raio));
    }

And in the main class:

public void main (String args[]) {
    Projeto p = new Projeto();
    System.out.println(p.Area(21.43, 33.4, 2.0));
   }

See working on ideone.

---

Another solution would be to change the classes and methods to static:

Class Areapiscina:

    public static class AreaPiscina {

    static double calcularArea(double raio){ 
      return((raio >= 0) ? Math.PI * Math.pow(raio, 2) : -1);
    }

}

Classe Areacasa:

public static class AreaCasa {
    //preço do metro quadrado
    static double valorM2 = 1500;

    //calcula a área da casa
    static double CasaRet(double l_sala, double c_quarto) {
        double area_s; //área da sala
        double area_q; //área do quarto
        double area_t = 0; //área do total

        if(l_sala < 0 || c_quarto < 0)
             System.out.println("Erro!");

        else{
            area_s = l_sala * l_sala; //calcula area da sala
            area_q = c_quarto * (l_sala/2); //calcula area do quarto            
            area_t = area_s + 2 * area_q; //calcula a area total
        }
        return(area_t);
    }
}

Your Design class and method Area:

public class Projeto {
 static double Area(double lateral_1, double lateral_2, double pis_raio) {
        return(AreaCasa.CasaRet(lateral_1, lateral_2) + AreaPiscina.calcularArea(pis_raio));
    }

    public static void main (String[] args) {
      System.out.println(Area(21.43, 33.4, 2.0));
    }
}

Functioning in the IDEONE.

Here are some links from SOPT for reading:

What is the function of a static method?

What is the use of a static or final variable in java?

Answer 2

static double CasaRet(double l_sala, double c_quarto) {
    double area_s; //área da sala
    double area_q; //área do quarto
    double area_t = 0; //área do total

    if(l_sala < 0 || c_quarto < 0)
         System.out.println("Erro!");

    else{
        area_s = l_sala * l_sala; //calcula area da sala
        area_q = c_quarto * (l_sala/2); //calcula area do quarto            
        area_t = area_s + 2 * area_q; //calcula a area total
    }
    return(area_t);
}

All we had to do was add the static in the method. Otherwise you should instantiate a class object and so call the method.

AreaCasa ac = new AreaCasa();
ac.CasaRet(valor1, valor2);